It has been recognised that obesity is a disease process influenced by environmental factors in which the traditional weight loss methods of dieting and exercise need to be supplemented by therapeutic products (S. Parker, “Obesity: Trends and Treatments”, Scrip Reports, PJB Publications Ltd, 1996).
Whether someone is classified as overweight or obese is generally determined on the basis of their body mass index (BMI) which is calculated by dividing body weight (kg) by height squared (m2). Thus, the units of BMI are kg/m2 and it is possible to calculate the BMI range associated with minimum mortality in each decade of life. Overweight is defined as a BMI in the range 25–30 kg/m2, and obesity as a BMI greater than 30 kg/m2. There are problems with this definition in that it does not take into account the proportion of body mass that is muscle in relation to fat (adipose tissue). To account for this, obesity can also be defined on the basis of body fat content: greater than 25% and 30% in males and females, respectively.
As the BMI increases there is an increased risk of death from a variety of causes that is independent of other risk factors. The most common diseases with obesity are cardiovascular disease (particularly hypertension), diabetes, including Type I and Type II diabetes, (obesity aggravates the development of diabetes), gall bladder disease (particularly cancer) and diseases of reproduction. Research has shown that even a modest reduction in body weight can correspond to a significant reduction in the risk of developing coronary heart disease.
Compounds marketed as anti-obesity agents include Orlistat (XENICAL®) and Sibutramine. Orlistat (a lipase inhibitor) inhibits fat absorption directly and tends to produce a high incidence of unpleasant (though relatively harmless) side-effects such as diarrhoea. Sibutramine (a mixed 5-HT/noradrenaline reuptake inhibitor) can increase blood pressure and heart rate in some patients. The serotonin releaser/reuptake inhibitors fenfluramine (Pondimin®) and dexfenfluramine (Redux™) have been reported to decrease food intake and body weight over a prolonged period (greater than 6 months). However, both products were withdrawn after reports of preliminary evidence of heart valve abnormalities associated with their use. There is therefore a need for the development of a safer anti-obesity agent.
The non-selective 5-HT2C receptor agonists/partial agonists m-chlorophenylpiperazine (mCPP) and trifluoromethylphenylpiperazine (TFMPP) have been shown to reduce food intake in rats (G. A. Kennett and G. Curzon, Psychopharmacol., 1988, 96, 93–100; G. A. Kennett, C. T. Dourish and G. Curzon, Eur. J. Pharmacol., 1987, 141, 429–435) and to accelerate the appearance of the behavioural satiety sequence (S. J. Kitchener and C. T. Dourish, Psychopharmacol., 1994, 113, 369–377). Recent findings from studies with mCPP in normal human volunteers and obese subjects have also shown decreases in food intake. Thus, a single dose of mCPP decreased food intake in female volunteers (A. E. S. Walsh et al, Psychopharmacol., 1994, 116, 120–122) and decreased the appetite and body weight of obese male and female subjects during subchronic treatment for a 14 day period (P. A. Sargeant et al., Psychopharmacol., 1997, 133, 309–312). The anorectic action of mCPP is absent in 5-HT2C receptor knockout mutant mice (L. H. Tecott et al., Nature, 1995, 374, 542–546) and is antagonised by the 5-HT2C receptor antagonist SB-242084 in rats (G. A. Kennett et al., Neuropharmacol., 1997, 36, 609–620). It seems therefore that mCPP decreases food intake via an agonist action at the 5-HT2C receptor.
Other compounds which have been proposed as 5-HT2C receptor agonists for use in the treatment of obesity include the substituted 1-aminoethyl indoles disclosed in EP-A-0655440. CA-2132887 and CA-2153937 disclose that tricyclic 1-aminoethylpyrrole derivatives and tricyclic 1-aminoethyl pyrazole derivatives bind to 5-HT2C receptors and may be used in the treatment of obesity. WO-A-98/30548 discloses aminoalkylindazole compounds as 5-HT2C agonists for the treatment of CNS diseases and appetite regulation disorders. 2-(2,3-Dihydro-1H-pyrrolo[1,2-a]indol-9-yl)ethylamine is disclosed in J. Med. Chem., 1965, 8, 700. The preparation of pyrido[1,2-a]indoles for the treatment of cerebrovascular disorders is disclosed in EP-A-0252643 and EP-A-0167901. The preparation of 10-[(acylamino)ethyl]tetrahydropyrido[1,2-a]indoles as anti-ischemic agents is disclosed in EP-A-0279125.
It is an object of this invention to provide selective, directly acting 5HT2 receptor ligands for use in therapy and particularly for use as anti-obesity agents. It is a further object of this invention to provide directly acting ligands selective for 5-HT2B and/or 5-HT2C receptors, for use in therapy and particularly for use as anti-obesity agents. It is a further object of this invention to provide selective, directly acting 5-HT2C receptor ligands, preferably 5-HT2C receptor agonists, for use in therapy and particularly for use as anti-obesity agents.